The present invention relates to the field of optical microscopes, and, in particular, to a method and apparatus for the prevention of application of excessive force between the objective or objective housing of a microscope and the stage thereof.
Modern optical microscopes are generally equipped with an eyepiece or eyepieces which are mounted to a microscope. Multiple objective lenses are mounted to the microscope-body and are carried by a turret which allows each lens in turn to be rotated into a light path extending through the microscope body from the objective to the lenses in the eyepieces. The sample to be studied is generally placed on a stage which is likewise mounted to the body of the microscope at a position beneath the objective. Illumination is frequently provided by means of a lamp mounted below the stage which is positioned to shine upward through an aperture in the stage. Biological microscopes typically require the sample to be mounted on a glass slide which is in turn placed on the stage where it may be held in place by clips. To focus the image, the distance between the objective and the stage is adjusted. This is generally accomplished by raising and lowering the stage, but is sometimes accomplished by raising and lowering the objective. For simplicity, the relative motion of the stage and objective will be discussed as raising and lowering of the stage, since raising and lowering of the objective produces essentially the same result.
The raising and lowering of the stage is generally accomplished by means of one or more focusing knobs located on the side of the microscope body. The knobs may include a fine adjustment knob and a coarse adjustment knob. The turning of the fine adjustment knob through a given angle results in less motion of the stage than the turning of the coarse adjustment knob.
When a sample is out of focus and the user is looking through the eyepiece or eyepieces of the microscope, the user does not see the fine detail of the sample. Rather, the user may only see a blurred and indistinct image. As the sample begins to come into focus, the blurring abates and the fine detail of the sample can be perceived. The transition from a blurred image to an in-focus image occurs over a rather short range of travel of the stage, particularly at high magnifications. As a result, finding the focus can be rather difficult, and it is not uncommon for a microscope user to turn the focus adjustment knobs in the wrong direction while seeking focus.
If the user raises the stage too high during the focusing process, contact between the objective and the slide may result. If too much pressure is applied at this point, the slide can break, the sample can be spoiled and the objective can be contaminated. The slide breakage problem is particularly acute with higher magnification objectives. This problem of slide breakage is longstanding, and affects even experienced microscope users.
Various means of reducing or eliminating the problem have been devised. One approach to avoiding the problem of slide breakage is the use of retractable objective carriers. According to this approach, the objective lenses of the microscope are mounted in carriers which are slidably mounted in the objective housings. Other lenses may be fixedly mounted in the objective housing. A spring acts between the objective housing and the carrier to bias the carrier to its fully downwardly-extended position. Flanges or stops on the objective housing and carrier retain the carrier within the objective housing.
If a user of such a microscope raises the stage to the point where the carrier touches the slide, the carrier retracts into the objective housing. Generally, such carriers are capable of being retracted up to about xc2xc inch (6.4 mm). At this point, the carrier reaches an internal stop which prevents it from retracting further into the objective housing. Such retractable objective carriers are seldom employed with lower power objectives, such as 2xc3x97 through 10xc3x97 objectives. Occasionally, they are found on 20xc3x97 objectives, but, more commonly, they are used only on objectives of 40xc3x97 or 100xc3x97 or greater. The reason for this is that the higher power lenses typically have a much shorter working distances. For example, the working distance of a 100xc3x97 objective is about 0.6 mm. Thus, even a minor amount of adjustment of the stage can result in breakage of the slide. As a result, the retractable objective carrier has not provided a complete solution to the slide breakage problem.
Another approach that has been taken is the providing of mechanical stops that limit the upward travel of the stage. Such stops may work in cooperation with the fine and coarse adjustment mechanisms, and are generally adjustable. The stop is generally intended to be set to prevent the coarse adjustment mechanism from raising the stage above a user-selected point. Typically, manuals for microscopes having such stops suggest that the stops be set using the highest power objective, which is typically 100xc3x97. The sample is first placed in focus at this setting, and the stop is then set. Since most modern microscopes are parfocal (all lenses focus on a given sample at the same stage elevation), no lens should need to have the stage raised beyond this point. This provides the two benefits of reducing slide breakage and allowing the focusing the microscope a single time, and achieve focus with any lens by adjusting the coarse focus until the stop is reached. Stops are generally not associated with the fine focus.
Unfortunately the use of stops has not eliminated slide breakage. First, few users know how to adjust the stops properly. In addition, many users who are familiar with the stop mechanism choose not to take the time to set it up and use it. Those users who do set the slide mechanism properly frequently do not check the adjustment on a daily basis. Finally, commercially-available slides vary in thickness, as do samples that are to be observed. As a result, frequent resetting of the stop would be necessary for each sample to ensure that slide breakage is avoided. Even if the stop is properly set, it should be noted that current stops only affect the coarse adjustment. Slide breakage frequently occurs as a result of use of the fine adjustment mechanism.
Thus, there remains an unfulfilled need for a system that will prevent slide breakage during adjustment of the height of the stage.
The present invention provides a method and apparatus for reducing slide breakage due to raising of the stage into contact with an objective. This is accomplished by the use of retractable objective carriers slidably mounted in the objective housings. A proximity sensor is positioned above the carrier, and is configured to detect the proximity of the carrier before the upper stop is reached. When the proximity sensor detects the proximity of the carrier (or an extension of the carrier), the system is activated to emit an audible signal and to extinguish the stage light that illuminates the sample. The extinguishing of the light serves as an indication that the stage has been raised too high. Further attempts to focus the microscope on the sample by further raising the stage are discouraged by the fact that the sample generally is not sufficiently visible for focusing once the light has been extinguished.